Hello, I'm a 18 year old girl and not active constantly. Lately my legs and feet have been very sore for the past couple days. What could this be?

Many options. Healthy young people can have muscle fatigue which should resolve over a week or so, from increased activity or injury. If it persists, you should consider stretching twice daily for 5-10 min. Otherwise, see a doctor for a more detailed exam.
Characterize aching. You question suggests you may be deconditioned - ie out of shape. If you have been active recently this may be muscle aching to out of shape muscles. Alternatively there can be a variety of reasons from the soft tissues - muscles, tendons to joints. We need to know what affects the aching, what releives the aching and what the physicial findings are. Are there other health problems?